The present invention relates to a method of purifying exhaust gas of an internal combustion engine by using a catalytic converter.
In exhaust gas purifying systems for purifying the gas exhausted from the combustion chambers by providing a catalytic converter in the exhaust systems of internal combustion engines, it is advantageous, from the standpoint of increasing the efficiency of purifying the exhaust gas, to confine the air-fuel ratio of the exhaust gas introduced into the catalytic converter within a predetermined range required by the catalytic converter and, in many cases, to confine the air-fuel ratio within a predetermined range around the stoichiometric air-fuel ratio. According to a conventional exhaust gas purifying system, an air-fuel ratio sensor is installed in the exhaust system to detect the exhaust gas air-fuel ratio in the engine by detecting the concentrations of particular components contained in the exhaust gas, then a carburetor installed in the intake system is controlled, in response to the detected results, in such a manner that the air-fuel ratio of the mixture gas supplied to the combustion chambers approaches the stoichiometric air-fuel ratio. According to another conventional exhaust gas purifying system, the air-fuel ratio of the mixture gas supplied into the combustion chambers is adjusted, in advance, to the rich side with respect to the stoichiometric air-fuel ratio, then, the secondary air is supplied into the gas exhausted from the combustion chambers depending upon the detected results of the air-fuel ratio sensor, in such a manner that the air-fuel ratio of the gas introduced into the catalytic converter lies within a predetermined range.
With the former method, the air-fuel ratio of the gas supplied into the combustion chamber can be controlled within a range around the stoichiometric air-fuel ratio, whereby unburned components exhausted from the combustion chamber are reduced, and thus, the temperature rise of the catalytic converter is restrained. When the engine is running at low speeds or when small loads are exerted on the engine, however, the feedback control for the air-fuel ratio is poor in accuracy, giving rise to the occurrence of variation in the air-fuel ratio and diminishing the operational characteristics of the vehicle. Furthermore, when the air-fuel ratio of the mixture gas supplied to the combustion chambers lies in the vicinity of the stoichiometric air-fuel ratio, the NOx components in the exhaust gas are increased.
With the latter method, on the other hand, the feedback control for the air-fuel ratio is performed in the exhaust system with high precision, which presents an advantage with regard to purifying the emission. Moreover, since the air-fuel ratio of the mixture gas supplied to the combustion chamber is rich in comparison with the stoichiometric air-fuel ratio, emitted NOx components are reduced, and high power output is produced by the engine, contributing to high operational performance of the vehicle. However, unburned components are contained in large amounts in the gas exhausted from the combustion chambers and, thus, the supply of the secondary air in amounts sufficient for the combustion of the unburned components often causes the catalytic converter to be overheated.